LED lighting technology is rapidly developing to replace traditional incandescent and fluorescent lightings. LED tube lamps are mercury-free in comparison with fluorescent tube lamps that need to be filled with inert gas and mercury. Thus, it is not surprising that LED tube lamps are becoming a highly desired illumination option among different available lighting systems used in homes and workplaces, which used to be dominated by traditional lighting options such as compact fluorescent light bulbs (CFLs) and fluorescent tube lamps. Benefits of LED tube lamps include improved durability and longevity and far less energy consumption; therefore, when taking into account all factors, they would typically be considered as a cost effective lighting option.
Currently, LED tube lamps used to replace traditional fluorescent lighting devices can be primarily categorized into two types. One is for ballast-compatible LED tube lamps, e.g., T-LED lamp, which directly replace fluorescent tube lamps without rewiring the lighting fixture; and the other one is for ballast-bypass LED tube lamps, which omit traditional ballast on their circuit and directly connect the commercial electricity to the LED tube lamp. The latter LED tube lamp is suitable for the new surroundings in fixtures with new driving circuits and LED tube lamps. The ballast-compatible type LED tube lamp is also known as “Type-A” LED tube lamp, and the ballast-bypass type LED tube lamp provided with a lamp driving circuit is also known as a “Type-B” LED tube lamp. Compared to the ballast-compatible type LED tube lamp, the ballast-bypass type LED tube lamp has better luminous efficacy and longer life time since the power consumption and the malfunction concerns of the ballast can be excluded.
For the ballast-bypass type LED tube lamp, the power supply configuration can be categorized into two types. One is single-end power supply configuration, which receives the external AC signal merely through one side of the LED tube lamp; and the other one is dual-end power supply configuration, which receives the external AC signal through both sides of the LED tube lamp. In order to fulfill the light emitting requirements of traditional fluorescent lamps, the circuits of the traditional fluorescent lamp fixtures are designed and disposed for providing the AC signal through both ends of the lamp. For the purpose of replacing traditional fluorescent lamps, an LED tube lamp having the dual-end power supply configuration can be popularized much easier since the installation is simpler than the single-end power supply configuration.
However, there still are some drawbacks in the dual-end power supply configuration. For example, when an LED tube lamp has an architecture with dual-end power supply and one end cap thereof is inserted into a lamp socket but the other is not, an electric shock situation could take place for the user touching the metal or conductive part of the end cap which has not been inserted into the lamp socket.
In the published application US 2013/0335959, filed on Jun. 15, 2012, a solution of disposing a mechanical structure on the end cap for preventing electric shock is proposed. In this electric shock protection design, the connection between the external power and the internal circuit of the tube lamp can be cut off or established by the mechanical component's interaction/shifting when a user installs the tube lamp, so as to achieve the electric shock protection. However, due to the physical characteristics of the mechanical components, the mechanical fatigue may inevitably cause the reliability and durability of the electric shock protection to be limited.
On the other hand, although the ballast-bypass type and the ballast-compatible type LED tube lamps can be configured in the dual-end power supply configuration, there still are many different considerations in the power supply circuit design. For example, due to the frequency of the voltage provided from the ballast being much higher than the voltage directly provided from the commercial electricity/AC mains, the skin effect occurs when the leakage current is generated in the ballast-compatible type LED tube lamp, and thus the human body would not be harmed by the leakage current. Therefore, since the ballast-bypass type LED tube lamp has higher risk of electric shock/hazard, compared to the ballast-compatible type, it is preferred that the ballast-bypass type LED tube lamp have extremely low leakage current for meeting strict safety requirements.
In the PCT patent application WO2015/066566, filed on Oct. 31, 2014, a solution of utilizing an electronic switch in the power supply circuit for preventing electric shock is proposed. In this electric shock protection design, a transistor/switch is disposed in series with the input rectification stage of the fluorescent lamp replacement and the LED load, and a current flowing through the sense resistor will be detected for determining whether the fluorescent lamp replacement is correctly connected to the ballast. WO2015/066566 addresses the electric shock protection in the ballast-compatible type LED tube lamp, however, it does not address the electric shock problem in the ballast-bypass type LED tube lamp.
In detail, compared to the power supply (typically an AC powerline or commercial electricity) for a ballast-bypass type LED tube lamp, the signal provided by a ballast (especially electronic ballast) has relatively high frequency or voltage. Further, for purposes such as one of driving a filament of a fluorescent lamp, a ballast may have to output a relatively high starting voltage for exciting electrons from the filament. So the starting voltage from a ballast can be as high as 1200 volts. On the other hand, the ballast-bypass type LED tube lamp is typically powered by commercial electricity with frequency as low as e.g. 50 Hz or 60 Hz and voltage as low as or below about 300 volts. Based on the above characteristics difference between power supplies for the direct replacement type LED tube lamp and the ballast-bypass type LED tube lamp, the benchmark and behavior for detecting the installation state is significantly different between the two types of LED tube lamp. For example, since the waveform of the current flowing through the sense resistor may be significantly different between the two types of LED tube lamp, utilizing the same determination criteria to determine whether the LED tube lamp is correctly installed is ineffective and will likely result in incorrect or inaccurate detection results. Thus, if the shock hazard detection of WO2015/066566 is applied to the ballast-bypass type LED tube lamp, a wrong detection result is relatively likely to occur, for example, because of the offset of the input voltage/current that may occur for lower frequency power signals.
Further, according to the circuit structure of WO2015/066566, a bias circuit is configured for starting the shock hazard detection, in which the input terminals of the bias circuit are connected to the ballast output at one side of the fixture. Therefore, the bias circuit can form a loop with the ballast and be powered up when one end of the LED tube lamp is installed on the corresponding socket of the fixture. However, since there is only one output in each side of the fixture for providing the dual-end power so that the loop of the bias circuit cannot be formed, the shock hazard detection circuit of WO2015/066566 cannot be implemented in most of the ballast-bypass type LED tube lamps.